Remembering the Classics: Cyanosis-Do You Think the Blood Looks Dark?
Alan Jay Schwartz, MD, MSEd
When I started my anesthesia residency there were recurring themes that characterized how we learned clinical care. As an example, when the skin incision was made on a patient under general anesthesia, it was a badge of a resident’s achievement if the surgeon didn’t blurt out, “The patient moved!”. How ironic it was that we as anesthesiologists were “blamed” for the patient response, when in fact, it was the surgeon who inflicted the pain.
Another routine we learned was to clinically assess the patient’s physiology by observing blood color upon skin incision. Appearance of dark purplish “black” blood was ominous indeed. Were this the case, there was obviously no need for a pulse oximeter to declare the high acuity of the patient’s state, yet little time was available to correct the malady.
The lore at the time was that clinicians could tell much about a patient’s oxygenation by observing cyanosis of the lips, nail beds and blood color as cyanosis began to appear. What is necessary for cyanosis to be present? Is it true that clinicians can accurately decern cyanosis? Can they observe this physiologic alteration in its early stages when there would be more time to rescue the patient?
Lundsgaard and Van Slyke1-3 studied and described the pathophysiology of cyanosis at the beginning of the 20th century. They described patient factors that influenced the presence of cyanosis. They demonstrated that there must be 5 gm/dL of reduced hemoglobin in 100 cc of capillary blood to produce visible cyanosis. This equated to an oxygen saturation of <85% and a PaO2 of approximately 60 mm Hg. They added that there must be sufficient hemoglobin to result in 5 gm/dL of reduced hemoglobin for cyanosis to be possible. Patients who are significantly anemic do not show cyanosis.
We were taught, therefore, that when oxygen saturation was 85% or less, a clinician can assume there would be cyanosis and the patient was hypoxemic. This was a clinical guide as early as the Spanish Flu epidemic of 19184. While the recognition of intense cyanosis had value identifying oxygenation deficit, a more moderate or mild blue-purple hue seemed a less reliable diagnostic clue.
It took almost 30 years from the pathophysiologic recognition that 5 gm/dL of reduced hemoglobin was necessary to produce visible cyanosis to acknowledge that its clinical assessment was often unreliable5. Comroe and Botelho were unconvinced stating, “It has been our impression for some years that excellent diagnosticians differ widely in their ability to recognize visually the presence of arterial anoxemia.”5 They asserted “…that the detection of cyanosis depends not only upon variables in the patient, but also upon variables in observers…” rendering “…cyanosis…a poor guide for the detection of arterial anoxemia of slight to moderate degrees.”5
To confirm their impression, Comroe and Botelho devised an elegantly simple study.5 Normal subjects inhaled a variety of low oxygen concentrations while their oxygen saturation was continuously monitored by oximetry. Clinical observers (105 medical students and 22 physicians including cardiologists and anesthesiologists) were asked to record when they observed if the study subjects were “normal”, “slightly or questionably cyanotic” or “definitely cyanotic”. Observers reported slight cyanosis >25% of the time when subjects were breathing room air or oxygen having oxygen saturations of 96%-100%. Notably, only 49% of the students and 53% of the physicians recorded definite cyanosis when the oximeter monitored 81-85% oxygen saturation. During instances when the oximeter recorded 71-75% saturation (an oxygen saturation of <75% indicating a PaO2 of approximately 40 mm Hg), 25% of the students and 15% of the physicians recorded only slight cyanosis. Twenty of the 127 observers did not note definite cyanosis until the oximetry was <75%. Eleven of the 127 noted definite cyanosis at oximetry readings >90%. Some observers were inconsistent in recording cyanosis in the same subject at different times when the oximeter had not changed.
In many cases, arterial anoxemia was unrecognized clinically until the saturation of hemoglobin with oxygen had fallen below 85% and in some it was unrecognized even at the 70% to 75% level
Comroe and Botelho stressed, “In present practice of medicine [1947], every effort is being made to place therapy upon a scientific basis. It is often essential to know blood…levels of [medications] in order properly to evaluate therapy. Yet in the evaluation of arterial anoxemia, the clinician still uses a relatively insensitive and unreliable guide, namely cyanosis…Since our experiments, we can infer that very few physicians are capable of detecting slight degrees of arterial anoxemia by the perception of surface blueness.”5
When I was a resident learning anesthesia patient care in 1973, pulse oximeters were not clinically available. The uphill climb from understanding the pathophysiology of cyanosis to the adoption of pulse oximetry into clinical practice was long indeed, ie, the 1920s to the 1980s. Invented in 1974, pulse oximeters thankfully became clinically available early in the 1980s.6
The educational principle that is operative and exampled by the clinical assessment of a patient’s cyanosis and its attendant deoxygenation is obvious, ie, employ clinical impression AND reinforce and validate it with quantitative data whenever possible.
References
1. Lundsgaard C: Studies on Cyanosis I. Primary Causes of Cyanosis. J Exp Med 1919: 30: 259-269
2. Lundsgaard C: Studies on Cyanosis II. Secondary Causes of Cyanosis. J Exp Med 1919: 30: 271-293
3. Lundsgaard C, Van Slyke DD: Cyanosis. Medicine 1923; 2: 1-76
4. Stadie WC: The Oxygen of the Arterial and Venous Blood in Pneumonia and its Related Cyanosis. J Exp Med 1919: 30; 215-240
5. Comroe JH, Botelho S: The Unreliability of Cyanosis in the Recognition of Arterial Anoxemia. The Amer J of the Med Sciences 1947; 214: 1-6
6. Miyasaka K, Shelley K, Takahashi S, Kubota H, Ito K, Yoshiya I, Yamanishi A, Cooper JB, Steward DJ, Nishida H, Kiani J, Ogino, Sata Y, Kopotic RJ, Jenkin K, Hannenberg A, Gawande A: Tribute to Dr. Takuo Aoyagi, Inventor of Pulse Oximetry. Journal of Anesthesia 2021; 35:671–709